Figure 6.1 “Water Molecules” shows that we require 2 hydrogen atoms and 1 oxygen atom to make 1 water molecule. If we want to make 2 water molecules, we will need 4 hydrogen atoms and 2 oxygen atoms. If we desire to make 5 molecules of water, we need 10 hydrogen atoms and also 5 oxygen atoms. The ratio of atoms we will certainly must make any variety of water molecules is the same: 2 hydrogen atoms to 1 oxygen atom.

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Figure 6.1 Water Molecules. The proportion of hydrogen atoms to oxygen atoms offered to make water molecules is constantly 2:1, no issue how many water molecules are being made.

One difficulty we have, but, is that it is incredibly hard, if not impossible, to organize atoms one at a time. As declared in the introduction, we address billions of atoms at a time. How deserve to we store track of so many atoms (and molecules) at a time? We perform it by utilizing mass quite than by counting individual atoms.

A hydrogen atom has a mass of approximately 1 u. An oxygen atom has actually a mass of approximately 16 u. The ratio of the mass of an oxygen atom to the mass of a hydrogen atom is therefore around 16:1.

If we have actually 2 atoms of each aspect, the proportion of their masses is around 32:2, which reduces to 16:1—the very same ratio. If we have 12 atoms of each element, the proportion of their total masses is roughly (12 × 16):(12 × 1), or 192:12, which additionally reduces to 16:1. If we have actually 100 atoms of each facet, the proportion of the masses is roughly 1,600:100, which aget reduces to 16:1. As long as we have actually equal numbers of hydrogen and also oxygen atoms, the proportion of the masses will constantly be 16:1.

The very same consistency is seen when ratios of the masses of various other elements are compared. For instance, the ratio of the masses of silicon atoms to equal numbers of hydrogen atoms is always roughly 28:1, while the ratio of the masses of calcium atoms to equal numbers of lithium atoms is around 40:7.

So we have establimelted that the masses of atoms are consistent with respect to each various other, as long as we have the very same variety of each kind of atom. Consider a more macroscopic instance. If a sample has 40 g of Ca, this sample has the exact same variety of atoms as tbelow are in a sample of 7 g of Li. What we need, then, is a number that represents a convenient quantity of atoms so we deserve to relate macroscopic amounts of substances. Clearly on even 12 atoms are also few because atoms themselves are so small. We need a number that represents billions and billions of atoms.

Chemists use the term mole to reexisting a large variety of atoms or molecules. Just as a dozen implies 12 things, a mole (mol) represents 6.022 × 1023 things. The number 6.022 × 1023, referred to as Avogadro’s number after the 19th-century chemist Amedeo Avogadro, is the number we usage in chemisattempt to represent macroscopic quantities of atoms and molecules. Hence, if we have 6.022 × 1023 O atoms, we say we have actually 1 mol of O atoms. If we have 2 mol of Na atoms, we have 2 × (6.022 × 1023) Na atoms, or 1.2044 × 1024 Na atoms. Similarly, if we have actually 0.5 mol of benzene (C6H6) molecules, we have 0.5 × (6.022 × 1023) C6H6 molecules, or 3.011 × 1023 C6H6 molecules.


A mole represents an extremely big number! If 1 mol of quarters were stacked in a column, it might stretch ago and forth between Planet and the sunlight 6.8 billion times.

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Notice that we are applying the mole unit to various kinds of chemical entities. In these examples, we cited moles of atoms and moles of molecules. The word mole represents a number of things—6.022 × 1023 of them—but does not by itself specify what “they” are. They can be atoms, formula units (of ionic compounds), or molecules. That information still demands to be specified.

Because 1 H2 molecule contains 2 H atoms, 1 mol of H2 molecules (6.022 × 1023 molecules) has actually 2 mol of H atoms. Using formulas to indicate just how many atoms of each aspect we have in a substance, we have the right to relate the number of moles of molecules to the variety of moles of atoms. For instance, in 1 mol of ethanol (C2H6O), we can construct the following relationships (Table 6.1 “Molecular Relationships”):

Table 6.1 Molecular Relationships1 Molecule of C2H6O Has1 Mol of C2H6O HasMolecular Relationships
2 C atoms2 mol of C atoms frac2,mol,C,atoms1,mol,C_2H_6O,molecules or frac1,mol,C_2H_6O,molecules2,mol,C,atoms
6 H atoms6 mol of H atoms frac6,mol,H,atoms1,mol,C_2H_6O,molecules or frac1,mol,C_2H_6O,molecules6,mol,H,atoms
1 O atom1 mol of O atoms frac1,mol,O,atoms1,mol,C_2H_6O,molecules or frac1,mol,C_2H_6O,molecules1,mol,O,atoms

Example 1

If a sample is composed of 2.5 mol of ethanol (C2H6O), how many type of moles of carbon atoms, hydrogen atoms, and oxygen atoms does it have?


Using the relationships in Table 6.1 “Molecular Relationships”, we apply the appropriate convariation factor for each element:

Keep in mind exactly how the unit mol C2H6O molecules cancels algebraically. Similar equations have the right to be built for determining the variety of H and O atoms:

2.5, lap————————mol,C_2H_6O,molecules,×,frac6,mol,H,atoms1, lap—————————mol,C_2H_6O,molecules,=,15,mol,H,atoms

2.5, lap————————mol,C_2H_6O,molecules,×,frac1,mol,O,atoms1, lap—————————mol,C_2H_6O,molecules,=2.5,mol,O,atoms

Example 2

How many formula devices are present in 2.34 mol of NaCl? How many type of ions are in 2.34 mol?


Usually in a problem like this, we start with what we are provided and also use the appropriate conversion variable. Here, we are provided a amount of 2.34 mol of NaCl, to which we deserve to apply the definition of a mole as a conversion factor:

2.34, lap———-mol,NaCl,×,frac6.022,×,10^23,NaCl,units1, lap————-mol,NaCl,=,1.41,×,10^24,NaCl,units

Because tbelow are two ions per formula unit, there are

1.41×10^24, lap————-NaCl,units,×,frac2,ions lap————-NaCl,units,=,2.82,×,10^24,ions